Not All Multiplexing Technologies Are on the Same Wavelength

Until recently, the development
of dense wavelength division multiplexing (DWDM) technology has focused solely on
scaling to higher channel counts, following pundits’ predictions that future
high-bandwidth applications would create a bandwidth shortage. This shortage has
proved largely illusory so far, and DWDM development has had to adapt to meet additional
demands.

One example is reconfigurable networks, which
provide flexibility and fast bandwidth, enabling carriers to realize a return on
their investment in infrastructure. The new and significant demands that reconfigurable
networks place on DWDM components have led to a variety of new multiplexing and
demultiplexing technologies.

Current DWDM options include thin-film
filters, fiber Bragg gratings, arrayed waveguide gratings and hybrid devices based
on free-space optics and diffraction gratings. The application of these technologies
is governed by a number of factors, including price (both initial cost and price
per channel), performance, footprint, channel counts, scalability, power consumption
and manufacturability.

Like arrayed waveguide gratings, free-space diffraction gratings multiplex light through
induced phase shift and interference. However, their passive operation frees them
from the need for heaters and enables much smaller components.High-channel-count applications

Major network hubs multiplex and demultiplex many
channels through single optical fibers — typically 40 channels for metropolitan
and 80 or more for long-haul hubs. This requires technology with high channel counts
and good uniformity.

Traditionally, thin-film filters coupled
by circulators (or more recently, interleavers) provide one approach to achieving
high channel counts. The efficiency of these devices does not rely on thermal compensation,
and their bandpass filter function is flat, which enables the layering of many filters
in a system.

However, they also present several
potential problems. First, although single thin-film filters have good optical properties,
when cascaded in series with other thin-film filters, circulators and interleavers,
their cumulative losses can impair overall DWDM system performance at high channel
counts. And although prices have dropped substantially, the economics of cascaded
thin-film filters are poor because interleavers, circulators and other coupling
technologies must be included in the overall system cost.

DWDM systems based on cascaded thin-film
filters are also bulky, often requiring 19-in. racks.

Alternative high-channel-count DWDM
filters use parallel processing, provided either by free-space diffraction gratings
or by arrayed waveguide gratings. These technologies separate all channels in a
single step rather than through a sequential series of filtering steps.

Parallel processing of light, provided either by free-space diffraction
gratings or by arrayed waveguide gratings, separates all channels in a single step
rather than through a sequential series of filtering steps.
The advantages are numerous. First,
these technologies allow high-channel counts without requiring additional coupling
components, which results in a much smaller footprint, lower cost, higher reliability
and increased optical performance.

Arrayed waveguide gratings, a relatively
new technology, multiplex light sources by causing the differential phase shifts
of “beamlets” to interfere. These devices have made great advances in
optical performance over the past few years, particularly with regard to insertion
loss. But in general, they are outperformed by free-space diffraction grating technology.

Free-space diffraction gratings also
separate light in a parallel fashion and operate using the same physical principles
as arrayed waveguide gratings: induced phase shift and interference. But free-space
diffraction gratings have the added advantage of being passive. That is, unlike
arrayed waveguide gratings, they do not require heaters to perform under all environmental
conditions. New athermal designs for arrayed waveguide gratings have been tested,
but their optical performance remains poor compared with heated versions.

Another advantage of free-space diffraction
gratings is their improved channel isolation, which results from a more accurate
phase shift imparted by the grating. The technology also offers better channel accuracy,
lower polarization-dependent loss and lower chromatic and polarization mode dispersion.
This last benefit stems from the fact that the free space within the device incurs
no birefringence.

Finally, free-space diffraction grating
designs are often “single-ended,” in that fibers enter and exit on the
same side of the device. This can enable efficient board layouts because it must
accommodate only one fiber bend radius.

One issue common to all dispersive
technologies — including both arrayed waveguide gratings and free-space diffraction
gratings — is that they naturally produce a Gaussian passband shape. When
coupled with other devices having a Gaussian passband, the effective filter function
narrows to the point where the system is incapable of high-data-rate transmission.
Flattop versions of both technologies have appeared that achieve a desirable filter
function with an approximately 2- to 3-dB penalty in insertion loss. Such devices
outperform the corresponding optical performance parameters for high-channel-count
subsystems based on thin-film-filters.

Low-channel-count applications

Another DWDM application involves add/drop sites
or nodes. Current network architectures use fixed add/drop functionality, in which
a particular node drops the same channels with no possibility of reconfiguration
without physical intervention on site. Nominally, the number of channels dropped
at a node varies from one to eight, but typical sites drop either four or eight
channels. Whatever the channel count, the dropped wavelengths require demultiplexing
from the network and a multiplexer to add channels on the main fiber. Thus, there
is a significant need for low-channel-count devices.

Here the advantage of free-space diffraction
gratings over thin-film filters and fiber Bragg gratings is less clear-cut. Low-channel-count
applications dilute the ability of free-space diffraction gratings (and arrayed
waveguide gratings) to amortize the fixed cost of the optics and packaging over
a large number of fibers. Meanwhile, thin-film filters can be made relatively inexpensively
and can perform well in applications with, say, fewer than 16 channels. However,
at least two emerging grating technologies have the potential to address this issue.

Dual-input free-space diffraction gratings
— as demonstrated by Zolo Technologies — share fixed costs of packaging
and optics over two and potentially more multiplexer/demultiplexers housed in the
same package. This reduces the cost per channel up to a factor of two compared with
separately housed multiplex assemblies. It also reduces the footprint by a factor
of two. Thus, a dual-input eight-channel device has approximately the same performance
and cost per channel as a single 16-channel device.

Another free-space diffraction grating
technology that could improve per-channel costs for low-channel count applications
was developed at the National Research Council in Ottawa. It is a hybrid technology
using a curved, etched echelle grating that couples light into and out of the device
via planar waveguides.

It has two potential advantages for
low-channel-count applications. First, the device has only two optical components
— the grating and the waveguide coupler — making it likely that the
technology will be inexpensive even at low channel counts. Second, because the device
is very small, its packaging costs may be reduced.

However, issues remain. Although the
yield on the etched gratings has reportedly improved over the last few years, very
high yields will be critical to maintaining low product costs. Also, these devices
are not passive; they require temperature control for proper operation, which increases
cost and the potential for failure.

Functional integration

Future networks will utilize a different, more
flexible architecture, particularly at the add/drop sites. Network architects covet
the ability to provision bandwidth according to demand, and this could fuel development
of reconfigurable add/drop multi- and demultiplexing at the network nodes. Such
technology would enable the network operator, with a software command, to increase
or decrease the number of channels dropped and added at any site at any time. Although
some companies now offer reconfigurable add/drop multiplexers, the devices typically
comprise individual components. Consequently, they are expensive, bulky and prone
to failure.

A great debate brews. What is the best
means of achieving higher-level, functionally integrated devices such as reconfigurable
add/drops and dynamic channel equalizers?

A few arrayed waveguide grating companies
offer package-level-integrated channel equalizers, but such devices have not yet
integrated the waveguide grating and attenuation function monolithically onto the
same silicon substrate. In fact, it could be argued that true integration will not
be possible or even desirable for many years.

There are two problems with true monolithic
integration using planar waveguide circuits. First, except for one or two companies,
arrayed waveguide grating yields are very low. A reconfigurable add/drop multiplexer
requires two (perhaps 40-channel arrayed waveguide gratings) on the same substrate
with a 2 x 2 switching technology displaced between the two waveguides for every
channel. The yield for such devices is not likely to hit double digits anytime soon.

Second, these devices would include
active components on a monolithic substrate (which also must be heated) with two
low-yield passive devices. So what happens when one of the active components fails?
The entire device must be scrapped. When these items cost a few hundred dollars
apiece (i.e., not per channel), scrapping failed devices may be an option. But it
is clearly not an option now.

Alternatively, functionally integrated
free-space diffraction grating devices rely on a paradigm that Zolo has already
demonstrated: reusing the same grating and optics to perform two separate multiplex
and demultiplex operations. Unlike arrayed-waveguide-grating-based reconfigurable
add/drops, free-space diffraction devices would require only a single grating.
The switching technology can be a simple one-dimensional array of microelectromechanical
systems mirrors. The array need have only as many mirrors as there are channels
on the network. This type of array can be made relatively inexpensively and with
good yields. Perhaps the greatest benefit is that free-space integrated devices
enable the ability to switch out the failed active component during the manufacturing
process, if not in the field.